I’m not sure if this has been suggested already but when looking at a tube map this afternoon (as you do) I noticed that some stations have tunnels that connect them so pedestrians can walk between stations that are close together? Wondered if this would work in Mini Metro. Could be given as an upgrade, allows passengers to get off at a station and get on at a connected station to shave a few minutes off journey time, is permanent once placed and has a limited range. Can only connect 2 stations at a time but stations can have multiple connectors connected to them. Example of pedestrian transfer below:

Yeah that sounds about right, I should also note it doesn’t increase capacity and of any stations, the concourse itself can transfer 6 passengers at a time (not sure about that rate of transfer, feel free to tinker with it)